An answer to a lunar mystery: Why is the moon’s gravity so uneven?

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Ever since the first satellites were sent to the moon to scout landing sites for Apollo astronauts, scientists have noticed a peculiar phenomenon: As these probes orbited the moon, passing over certain craters and impact basins, they periodically veered off course, plummeting toward the lunar surface before pulling back up.

As it turns out, the cause of such bumpy orbits was the moon itself: Over the years, scientists have observed that its gravity is stronger in some regions than others, creating a “lumpy” gravitational field. In particular, a handful of impact basins exhibit unexpectedly strong gravitational pull. Scientists have suspected that the explanation has to do with an excess distribution of mass below the lunar surface, and have dubbed these regions mass concentrations, or “mascons.”

Exactly how these mascons came to be has remained a mystery — until now.

Using high-resolution gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, researchers at MIT and Purdue University have mapped the structure of several lunar mascons and found that their gravitational fields resemble a bull’s-eye pattern: a center of strong, or positive, gravity surrounded by alternating rings of negative and positive gravity.

To figure out what caused this gravitational pattern, the team created simulations of lunar impacts, along with their geological repercussions in the moon’s crust and mantle, over both the short- and long-term. They found that the simulations reproduced the bull’s-eye pattern under just one scenario.

When an asteroid crashes into the moon, it sends material flying out, creating a dense band of debris around the crater’s perimeter. The impact sends a shockwave through the moon’s interior, reverberating within the crust and producing a counterwave that draws dense material from the lunar mantle toward the surface, creating a dense center within the crater. After hundreds of millions of years, the surface cools and relaxes, creating a bull’s-eye that matches today’s gravitational pattern.

This tumultuous chain of events likely gave way to today’s lunar mascons, says Maria Zuber, the E.A. Griswold Professor of Geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“For the first time, we have a holistic understanding of the process that forms mascons,” says Zuber, who is also GRAIL’s principal investigator, and MIT’s vice president for research. “There will be more details that emerge for sure, but the quality of the GRAIL data enabled rapid progress on this longstanding question.”

Zuber and her colleagues have published their results this week in Science.

Mapping a bumpy ride

From January to December 2012, GRAIL’s twin probes, Ebb and Flow, orbited in tandem around the moon, mapping its gravitational field by measuring the changing distance between themselves — a real-time indication of the strength of the moon’s gravitational pull. As the probes got closer to the moon’s surface toward the end of the mission, Zuber recalls, engineers had to adjust the probes’ orbits to counteract the tug of lunar mascons.

“Because the moon’s gravity field is so bumpy, we would put the two spacecraft in a circular orbit, and the orbits immediately became elliptical because the spacecraft got tugged out of their orbit,” Zuber says. “We were always within a week of crashing.”

Despite the impending threat of impact, the probes gathered high-resolution measurements, which Zuber and the GRAIL science team have since translated into detailed gravitational maps. These maps also gave scientists precise measurements of the thickness of lunar crust in any given region of the moon, which Purdue’s Jay Melosh integrated into impact simulations.

Melosh simulated the process of lunar impacts in two similarly sized basins on the near side of the moon — one with lava deposits, the other without. Melosh fed the crustal thicknesses from both basins into the model, then ran the simulation to see how the same impact would affect each region.

According to measurements from GRAIL, the basin containing central lava deposits had a thinner crust than the other basin. After running their simulations, the researchers found that an impact had created a gravitational bull’s-eye pattern in the first basin, but not the second — predictions that matched GRAIL’s measurements.

Making an impact

Why the difference in gravitational signatures? The answer, the group found, lay in the crust’s thickness at the time of impact: Impacts to regions with thinner crust do more damage, easily sending shockwaves into the denser, underlying mantle — which, in turn, draws more dense material to the surface, creating a mascon. Regions with thicker crust, by contrast, are more resistant to impacts and internal upheaval.

“Large impacts happen in seconds to hours,” Zuber says. “The process of how the crust cools off and recovers from such a devastating event, that’s hundreds of millions of years. So we let these models run through time until the surface cools and relaxes. Then what you’re left with is today’s gravity.”

The results from the group’s simulations precisely matched GRAIL’s actual gravity measurements, giving scientists confidence that the simulated impact scenario is indeed what formed the lunar mascons.

While most scientists agree that the moon’s mascons likely arose from large impacts, Laurent Montesi, an associate professor of geology at the University of Maryland, says the precise processes that led to the formation of the mascons has been a mystery since their discovery 45 years ago.

“This paper finally proposes an answer to this longstanding puzzle by including a start-to-finish model of mascon formation,” says Montesi, who did not contribute to this research. “It is now clear that geological processes occurring over millions of years are needed to turn the structure produced immediately by the impact into a mascon. It is remarkable how well the models in the paper reproduce the observed structures.”

Zuber says that knowing what gave rise to lunar mascons may help us understand the evolution of the moon, as well as other planets. The mascons likely formed during a period known as the Late Heavy Bombardment, when the early solar system endured a blitz of interplanetary collisions. The Earth may have undergone even more impacts than the moon, although the resulting craters have since been erased by erosion and plate tectonics. Studying the repercussions of impacts on the moon therefore might offer clues to Earth’s origins.

“This was a very inhospitable time to be at the surface of a planet,” Zuber says. “The tail end of this process is when the first single-celled organisms emerged on Earth. So knowing what the effect of the impacts was on the thermal state of a planet that early tells us about the extreme conditions under which life on Earth took hold."

Comments

george

June 3, 2013

Earth has also been hit by asteroids. Does earth also exhibit similar gravitational anomolies? Cough***bermuda triangle***cough.

scnehruroy

June 3, 2013

Planet Earth is a lively and unique and its' gravitational behaviour is incomparable.Moon being a satellite has its own making , in fact its' showing the same side to earth is bound to have gravitional effect. Although study on satellites of other planets may yield some clue, one cannot derive a definite answer owing to varying locational aspect of other celestial bodies.

Jai Aggarwal

June 10, 2013

Bermuda triangle is a great topic to think about as if it has to be big ocean waves they might or must lead waste to any part of earth...

But it is not so, so a detailed study should be made there about this type of anomaly...

Maria Perez

June 10, 2013

I believe earth's mass and atmosphere is incomparable to the moon's. Asteroids, most likely, upon entering the atmosphere of the earth, do hit the earth in a similar way as they may the moon. With the moon, the atmosphere is thin and the amount of land or mass occupying it is very much few in comparison to the earth's large mass. Also, I believe that, whether or not the "Bermuda Triangle" Phenomena has a scientific basis, the explanation cannot possibly be related to anomalies regarding gravity.

Rob

June 10, 2013

Gravitational anomalies can be found on earth, ask any Captain who will point you to charts of our oceans with such occurrences detailed. The most interesting aspect of this for me is that a spinning ball (moon) with a magnet on the equator would stop spinning if attracted to a nearby magnet source (earth) with the magnet facing the nearby magnetic source...this is most likely why our moon always faces us with the same side. The face we see of the moon has the greatest magnetic attraction to our earth.

Adele Cleverley

June 10, 2013

could this also be the cause of the large magnetic forces that cause magnetic declination (variation) from true north around the world..? i wonder..

Tero Tommola

June 10, 2013

Ahem, Bermuda Triangle is not a "gravitational anomaly", per se.

The gravitational anomaly would have to be massive in order to pull a ship down through the surface tensity of water.

A more plausible theory is gas bubbles from frozen methane deposits deep in the seabed, slowly melting and releasing sudden gushes of bubbles. The bubbles can lessen the surface tension of water, causing a ship of any size to sink almost instantaneously.

James O' Donoghue

June 10, 2013

@hoffm263 - The bermuda triangle doesn't have any more of less mystery than anywhere else on Earth. The reason it has a reputation for a higher accident rate is simply because there is more traffic going through the region. It's like saying your nearest highway has more accidents than your driveway and that it's obviously aliens, gravitational or magnetic phenomenon.

Johnny Utah

June 17, 2013

Surface tension isn't what keeps ships afloat. You can float a needle on top of smooth water due to surface tension, but as soon as the water is disturbed, the needle sinks.

Boats, on the other hand, stay afloat by being, in total, less dense than water.

You could cause a boat to sink with bubbles because they decrease the density of the water...then when the boat is submerged, water enters places it normally wouldn't (through hatches etc) increasing the relative density of the boat such that even without the bubbles, the boat is now more dense than water...and sinks.

Seal up a boat such that no water can enter and even if you pull it under water it'll pop back up like a cork.

If it were surface tension holding boats afloat, they wouldn't survive their first encounter with a wave.